Young adults with type 1 diabetes (T1D) commonly struggle with both glycemic control and weight. The prevalence of overweight and obesity in T1D parallels that of the general population. There is a compelling need to develop interventions to optimize two key outcomes ? glycemic control and weight status ? that address underlying metabolic processes and behavioral challenges unique to T1D. Accordingly, our main objective is to develop a rigorous adaptive design to test the efficacy of behavioral interventions to simultaneously optimize glycemic control and facilitate weight management among overweight young adults with T1D. In June 2015, we established a transdisciplinary consortium, ACT1ON: Advancing Care for Type 1 Diabetes and Obesity Network, at three institutions: University of North Carolina (UNC), Florida Hospital Translational Research Institute for Metabolism and Diabetes (TRI), and the Division of Pediatric Endocrinology and Diabetes at Stanford University. Together, we propose Specific Aims in three phases: Aim 1: Phase One (18 months): To establish a rigorous scientific foundation for this work. Study One: The Influence of Glycemic Control and Obesity on Energy Balance and Metabolic Flexibility in Type 1 Diabetes. Goal: To develop a comprehensive model of energy balance and metabolic flexibility in T1D that accounts for hyperglycemia and body composition. We will enroll 33 individuals at TRI (8 lean T1D, 8 overweight T1D, 8 obese T1D; with three age-, sex-, and weight-matched non-diabetic controls per group). Study Two: Dietary Response to Non-Severe Hypoglycemia. Goal: To explore the role of non-severe hypoglycemia on weight status and dietary intake, using data from a funded study (UC4DK101132 MPIs, Mayer-Davis, Maahs, Seid). Aim 2: Phase Two (24 months): To conduct a pilot and feasibility study (n=72, 3 experimental diets, 9 months) using a sequential, multiple assignment, randomized trial (SMART) design to identify acceptable and effective dietary strategies to optimize both glycemic control and weight management in young T1D adults. Aim 3: Phase Three (6 months): To incorporate findings from Aims 1 and 2 to finalize the intervention strategy and SMART design of a fully-powered efficacy trial of behavioral interventions that will be acceptable and feasible to optimize glycemic control and weight management in young adults with T1D. We are uniquely qualified with expertise including clinical care, energy metabolism, nutrition, psychology, clinical trial design and statistical analyses, all with a focus on T1D. The proposed research is innovative, with state-of-the-art bioenergetic modeling, trial design, and statistical methods. Achievement of our objective would be highly significant and will ultimately lead to changes in clinical practice with an effective, acceptable approach to co-manage glycemia and weight in young adults with T1D.